Reflector discharge lamp having frosted envelope and arc tube



May 21, 1968 J. M. POMFRETT REFLECTOR DISCHARGE LAMP HAVING FROSTED ENVELOPE AND ARC TUBE Filed Feb. 8,

, Invervtor. John M. Pom?e t=-t b5 &/ 7

His AttOT'TWeLd /0 a0 DEGREES F/eoM 850M flx/s 0 3 X WHQS Q E g United States Patent 3,384,771 REFLECTOR DISCHARGE LAMP HAVING FROSTED ENVELOPE AND ARC TUBE John M. Pomfrett, Kirtland, Ohio, assignor to General Electric Company, a corporation of New York Filed Feb. 8, 1965, Ser. No. 430,892 2 Claims. (Cl. 313-116) This invention relates to high pressure gas or vapor lamps of the reflector type wherein an arc tube is mounted in a glass bulb provided with a reflecting coating for concentrating the light in a desired direction or pattern.

Since the invention up to the present has been most useful in high pressure mercury vapor lamps, it will be described with reference thereto. In such lamps, the arc tube is made of quartz and is enclosed within a hard glass bulb or outer jacket which keeps air away from the arc tube seals and provides mechanical protection while serving also to conserve heat and filter out short wave ultraviolet radiation. The glass bulb is formed to a bowlshape extended at one end into a reduced diameter tubular neck portion to which is attached a screw type base accommodating in a standard socket. The are tube is mounted within the glass bulb with its longitudinal axis corresponding with that of the bulb. The bowl portion of the bulb is silvered to reflect light in a controlled beam aimed along the longitudinal axis.

In flood lighting applications, a medium flood candlepower distribution is desirable having a beam spread of about 70. The beam spread is measured from cutoff to cutoff through the axis, the cutoff being taken as the point where the candlepower is down to of the maximum. A lamp of this type which is commercially available from the assignee of this application bears manufacturers designation H40ORFL33-1 and is also designated under the American Standards Association code by H33-1LN. This is a 400-watt lamp utilizing a hard glass outer bulb or jacket commonly designated R-60. One source of supply for such bulbs is Corning Glass Works of Corning, NY. It has been the practice on the part of the lamp manufacturers to purchase the bulbs from the bulb manufacturer lightly frosted on the inside, such frosting being known under the designation Corning No. 5.

A criticism which has been leveled at these medium flood reflector lamps as made up to now is that the beam pattern has a very hot or bright spot along the axis. Also the illumination within the beam is lacking in uniformity as evidenced by alternating bright and dark bands in the beam pattern. It might be expected that a solution to these problems would reside in providing a heavier frost on the bulb face. However heavy frosting of a hard glass bulb is inordinately expensive and may cost as much as the original bulb itself. Also as the frost is made heavier or more diffusing, it becomes extremely diflicult to maintain uniformity of the product and achieve a constant degree of frosting or light diffusion. Variation in the frost affects the beam pattern and results in an unsatisfactory product.

The object of the invention is to provide a new and improved reflector type discharge lamp which solves the foregoing problems in economical fashion.

In accordance with the invention, I have found that a large reduction in hot spot and a substantial improvement in beam pattern may be effected by frosting the quartz arc tube. The are tube may conveniently be frosted after the electrodes have been scaled into it by subjecting it to a sandblasting. Preferably the sand-blasted arc tube is used in combination with a lightly frosted bulb. An unexpected benefit of the invention is an increase in measured beam lumens.

For further objects and advantages and for a better understanding of the invention, attention is now directed 3,384,771 Patented May 21, 1968 to the following description of a preferred embodiment and to the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is an elevation view of a reflector type high pressure mercury vapor lamp embodying the invention.

FIG. 2 is a graph comparing the candlepower distribution curve of the lamp of FIG. 1 with that of a prior reflector type lamp.

Referring to FIG. 1 of the drawing, the illustrated lamp comprises a glass bulb 1 made of a heat and weather-resistant hard glass such as a borosilicate glass. The bulb has a tubular reduced diameter neck portion 2 to the end of which is sealed a re-entrant stem 3 having a press 4 through which extend relatively stiff inlead Wires 5, 6. To the outer end of the neck is cemented the usual screw type base, the inleads 5, 6 being connected respectively to the threaded shell 8 and insulated center contact 9 of the base.

The inner end of the neck portion 2 extends into a flaring bowl portion 11. The glass bulb 1 is symmetrical and formed as a surface of revolution about the longitudinal projection (vertical) axis, the bowl portion being generally paraboloidal. The designation R-60 indicates that the maximum diameter of the reflector is 60% of an inch, namely 7 /2 inches. The bowl or reflector portion is coated with a light-reflecting layer 12, for instance an internal coating of silver, or alternatively of aluminum. The entire bulb, including the convex end face 13 and also the flaring bowl portion 11, is preferably lightly frosted on the inside, suitably with a Corning N0. 5

I frost. The frosting of the bowl portion under the reflective coating serves to diffuse reflected light, as does also the frosting of the end face through which the light is transmitted.

The inner arc tube 14 is made of quartz; it extends along the projection axis of the bulb and is approximately centered on the focus of the bowl portion, the exact position depending on the beam spread desired. It is provided at opposite ends with main discharge supporting electrodes 15, 16 supported on inleads hermetically sealed through the pinched ends 17 of the arc tube and including ribbon seals 18. Each main electrode comprises a core portion which may be a prolongation of the inner end of the inlead made of tungsten and surrounded by a double tungsten wire helix. The turns of the helix and the interstices between turns are coated with an electronemitting mixture of alkaline earth oxides. An auxiliary starting electrode 19 consisting merely of the inwardly projecting end of the inlead is provided at the base end of the arc tube adjacent main electrode 15. The auxiliary electrode is electrically connected to main electrode 16 at the opposite end of the arc tube through the usual current limiting resistor 20. The are tube contains a charge of mercury and an inert gas such as argon at a pressure below millimeters of mercury. As described for instance in the textbook High Pressure Mercury Vapour Lamps and Their Applications, edited by W. Elen'baas and published by Philips Technical Library, Eindhoven, Netherlands, 1965, high pressure mercury vapor lamps have the characteristic that, as mercury vapor pressure builds up, the discharge detaches itself from the tube wall and in normal operation at pressures of one atmosphere or more, becomes a narrow cord extending along the axis with a relatively dark annular space between the discharge and the walL-Chap. 1, Sec. 1.4.

The are tube is supported within the glass bulb by means of a frame or harness 21, suitably of nickel-iron alloy or nickel-plated iron wire. The harness is made from a continuous length of wire and comprises a principal vertical portion or side rod which extends parallel to the bulb axis and which is welded at its lower end to inlead 5. The arc tube is supported in the harness by clamping its flat pinched ends 17 between metal bands 22, 23 which extend between the side rod and short vertical wire portions 24, 25. At the upper end, the wire makes a half loop around a recess or dimple 26 of generally cylindrical configuration in the end face of the bulb. A springy split hexagonal metal clip 27 is attached to the harness at the half loop and engages the dimple to anchor the upper end of the harness.

In accordance with the illustrated embodiment of the invention, the outer surface of arc tube 14 is heavily frosted, as indicated in the drawing by speckling. This may conveniently be done by sandblasting; for instance the finished arc tube may be subjected to two passes under a blast of air-entrained sharp quartz sand, previous to mounting in the glass bulb. The frosting is heavy enough that the internal parts of the arc tube such as the electrodes are no longer discernible. Referring to FIG. 2, solid line curve 31 illustrates a typical candlepower distribution obtained with the lamp which has been described, using a sandblasted arc tube and a lightly frosted bulb. Dotted line curve 32 illustrates the candlepower distribution of a prior art lamp utilizing a clear quartz arc tube in a lightly frosted bulb. The curves indicate the candlepower measured at various angles of divergence from the beam axis. It will be observed that with the clear arc tube of the prior art, the maximum beam candlepower as seen along the axis is about 48,000 candles whereas with the frosted arc tube of the invention, the maximum beam candlepower has dropped to about 39,000. From about 2 to over 30 off axis, the solid line curve for the frosted arc tube is above the dotted line curve for the clear arc tube. This indicates a lesser hot spot and a more uniform candlepower distribution over the beam spread of the lamp using the frosted arc tube.

Since all of the light emitted by the lamp originates in the arc tube, one might anticipate that frosting the arc tube would reduce the total light output of the lamp. Surprisingly however when the total beam lumens, that is the beam lumens in the cone extending out 35 on either side of the projection axis, are measured it is found that frosting the arc tube increases the beam lumens from approximately 6800 to approximately 8800. The improvement is due to the greater uniformity of candlepower over the beam spread, that is reduction of the hot spot on the axis combined with increase in intensity over the major portion of the beam out to the cutoff points. Candlepower at angles remote from the axis is of course more significant in the integration to determine total beam lumens than candlepower next to the axis.

In life tests which I have conducted on lamps using sandblasted arc tubes in accordance with the invention, no appreciable reduction in life or maintenance characteristics has been observed. In view of the known tendency of quartz to devitrify at high temperatures from such minor causes as soiling due to handling, this is a rather surprising result at the same time as it is a very desirable one.

In a high pressure lamp, the arc tube is so bright that the eye is dazzled by it and the great variation in relative brightness throughout is easily overlooked. However by observing the arc tube through dark smoked glass, it is readily seen that the discharge occurs as a narrow constricted cord, at most a few millimeters in diameter, which extends along the projection axis of the lamp on either side of the focus. The rest of the discharge space is relatively dark. By frosting the arc tube, the apparent size of the light source is expanded from a narrow cord whose cross section is but a small fraction of that of the arc tube to the full diameter of the arc tube. The source then overlaps greately the focus of the reflector in all directions with the resultant desired diffusing effect.

As previously mentioned, for a medium flood lamp, I prefer to use a light inside frost known as Corning No. 5 on the bulb, both on the end face and on the bowl under the reflective coating, along with sandblasting of the arc tube. With this frost, the outlines of the arc tube are still discernible when seen through the end face of the glass bulb even though blurred. However if a beam with a somewhat hotter spot and less uniformity of distribution is acceptable, one may use a frosted arc tube in a clear bulb. I have found that the results in respect of hot spot and beam pattern when using a frosted arc tube in a clear bulb are roughly the same as when using a clear arc tube in a bulb provided with a light inside frost.

While the invention has been most useful to date in conjunction with high pressure mercury vapor lamps and has been so illustrated and described, it is applicable to any type of electric discharge lamp involving a dual envelope construction and wherein constriction of the are into a narrow cord occurs in the arc tube. The specific embodiment which has been illustrated and described in detail is intended of course as exemplary and the scope of the invention is to be determined by the appended claims.

I claim:

1. A reflector electric discharge lamp of the flood type comprising a glass bulb generally symmetrical about its longitudinal projection axis and having a tubular neck portion extending along said axis and opening into a flaring bowl portion closed by a light-transmitting convex end face, said bowlv portion being coated with a lightreflecting layer approximately up to its region of maximum diameter, a quartz arc tube mounted within said glass bulb and extending along the projection axis on either side of the focus, said are tube containing a discharge medium in which the arc discharge is constricted into a narrow cord whose cross section is but a small fraction of the cross section of the arc tube, said glass bulb being internally lightly frosted in both bowl and end face portions, and the external surface of said arc tube being heavily frosted in order to expand the apparent size of said luminous cord to the overall size of said are tube.

2. A lamp as in claim 1 wherein the frosted surfac of the arc tube is sandblasted.

References Cited UNITED STATES PATENTS 3,054,922 9/ 1962 Louden et a1. 313221 2,215,440 9/1940 Ruse 313-411 2,394,495 2/ 1946 Smith 313111 2,517,126 8/1950 Macksoud 313--111 2,545,896 3/1951 Pipkin 313116 2,749,461 6/ 6 Hierholzer et a1. 31325 2,759,119 8/ 1956 Thorington 313-25 2,830,210 4/1958 Jenne et al. 313-25 JAMES W. LAWRENCE, Primary Examiner.

ROBERT SEGAL, Examiner.

R. JUDD, Assistant Examiner. 

1. A REFLECTOR ELECTRIC DISCHARGE LAMP OF THE FLOOD TYPE COMPRISING A GLASS BULB GENERALLY SYMMETRICAL ABOUT ITS LONGITUDINAL PROJECTION AXIS AND HAVING A TUBULAR NECK PORTION EXTENDING ALONG SAID AXIS AND OPENING INTO A FLARING BOWL PORTION CLOSED BY A LIGHT-TRANSMITTING CONVEX END FACE, SAID BOWL PORTION BEING COATED WITH A LIGHTREFLECTING LAYER APPROXIMATELY UP TO ITS REGION OF MAXIMUM DIAMETER, A QUARTZ ARC TUBE MOUNTED WITHIN SAID GLASS BULB AND EXTENDING ALONG THE PROJECTION AXIS ON EITHER SIDE OF THE FOCUS, SAID ARC TUBE CONTAINING A DISCHARGE MEDIUM IN WHICH THE ARC DISCHARGE IS CONSTRICTED INTO A NARROW CORD WHOSE CROSS SECTION IS BUT A SMALL FRACTION OF THE CROSS SECTION OF THE ARC TUBE, SAID GLASS BULB BEING INTERNALLY LIGHTLY FROSTED IN BOTH BOWL AND END FACE PORTIONS, AND THE EXTERNAL SURFACE OF SAID ARC TUBE BEING HEAVILY FROSTED IN ORDER TO EXPAND THE APPARENT SIZE OF SAID LUMINOUS CORD TO THE OVERALL SIZE OF SAID ARC TUBE. 